The effectiveness of milk thistle in human liver disease has not been established. This may be because of the scientific quality of study methods or published reports or both. Possible benefit has been shown most frequently, but not consistently, for improvement in aminotransferases, but liver function tests are overwhelmingly the most common outcome measure studied. Survival and other clinical outcome measures have been studied least often, with both positive and negative findings. Mechanisms of action, disease populations likely to benefit, the optimal formulations of milk thistle, and duration of therapy are undefined. Also, little about adverse effects is known. Further study of mechanisms of action and well-designed clinical trials, with detailed reporting of adverse effects and components of potential causality, are needed.